Radio paging systems are increasingly becoming part of the infrastructure of our "information age" society. No longer is their use limited to summoning doctors and emergency response technicians. Instead, they are used for virtually all communications tasks. Commonplace applications for radio paging systems now include office paging (in lieu of public address paging) and relaying grocery lists to spouses on their ways home from work.
While radio paging systems have previously been local in nature, newer systems offer virtually instant point-to-point communications anywhere on the globe. U.S. Pat. No. 4,713,808 to Gaskill et al., the disclosure of which is incorporated herein by reference, is exemplary of such new global paging technology.
Before radio paging systems can become truly ubiquitous, the receiving technology used in the personal paging receivers must be improved, both in convenience and technological sophistication. Traditionally, paging receivers have been box-like devices that must be clipped to a belt. More recently, pocket pagers have been developed. However, both of these two approaches still require the user to carry an additional item on his or her person.
More promising is the approach disclosed in the Gaskill patent in which a pager is incorporated into a conventional electronic wristwatch. Although much more convenient than prior art paging receivers, the patented Gaskill system confronts several technical challenges.
One such challenge is the issue of shielding different sections of the receiver from one another. Due to space constraints, shielding in the wristwatch sized enclosure is essentially unobtainable. Since noise from one circuit cannot be isolated from others, it must be eliminated.
An example of the shielding problem is in the frequency control circuits. The Gaskill system relies on a frequency agile receiver that scans the FM broadcast spectrum for a signal having paging data on a subcarrier thereof. The frequency agility is effected by a microprocessor controlled oscillator, such as a fractional-N synthesizer. Such digital synthesizers are electrically quite noisy and are typically isolated from other circuitry by adequate shielding. In the wristwatch enclosure, however, such shielding cannot be achieved without substantial penalties in cost, size and weight. Nor can the frequency agility be achieved without the synthesizer.
In accordance with the present invention, this noise problem is overcome by operating the digital synthesizer only at the beginning of each listening interval. After the receiver has stabilized, the synthesizer is shut down and frequency control is thereafter assumed by an automatic frequency control (AFC) system that maintains the receiver on the desired frequency. In addition to eliminating synthesizer noise, this AFC operation also cuts power consumption by the receiver.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.